Researchers Crack XML Encryption Mechanism, Propose New Standard

A flaw in XML Encryption leaves major Web services vulnerable to potential attack unless Apache, Red Hat, IBM, Microsoft and other major XML framework providers adopt a new standard.

Security
researchers have cracked the major XML framework used to encrypt data in major
Web applications.
Two
researchers from Germany's Ruhr-University demonstrated a practical attack
against XML's cipher block chaining module at the ACM Conference on Computer
and Communications Security in Chicago Oct. 19. The technique affects messages
encrypted with any of the algorithms supported by the XML Encryption standard,
including the popular AES and DES.

"We
were able to decrypt data by sending modified ciphertexts to the server-by
gathering information from the received error messages," the researchers
said in a statement.

XML,
or eXtensible Markup Language, is used for storing and transporting data and is
widely used for Web applications such as business communications, e-commerce,
financial services, health care, and government and military infrastructure.
Standardized in 2002 by the World Wide Web Consortium (W3C), an Internet
standards group, XML Encryption is widely used by Apache, Red Hat, IBM and
Microsoft in their XML frameworks.
"There
is no simple patch for this problem," said Juraj Somorovsky, one of the
researchers, adding, "We therefore propose to change the standard as soon as
possible."
Researchers
proposed replacing the CBC module in XML Encryption with a mechanism focused on
both message confidentiality and message integrity.
Adopting
a new approach and changing the standard, however, would likely affect existing
deployments and create backward compatibility issues with older applications,
the researchers said.
A
potential attack vector involves sending bogus messages to a targeted system
and then using the information returned by the system to crack the encryption.
"We
show that an adversary can decrypt a ciphertext by performing only 14 requests
per plaintext byte on average," the researchers said. "This poses a
serious and truly practical security threat on all currently used
implementations of XML Encryption."
The
German team notified all affected XML framework providers, including
Amazon.com, IBM, Microsoft and Red Hat, via the W3C mailing list before
releasing its paper. It has engaged in "intensive discussions on
workarounds" with some of the affected organizations.
Amazon.com
acknowledged the issue on Oct. 20 and said it had fixed the related vulnerabilities
in XML-based messaging protocol Simple Object Access Protocol (SOAP) in its
Elastic Compute Cloud (EC2) infrastructure. The company also checked to ensure
no customers had been targeted by potential attackers.
"The
research showed that errors in SOAP parsing may have resulted in specially
crafted SOAP requests with duplicate message elements and/or missing
cryptographic signatures being processed," the company wrote in the Amazon
Web Services security bulletin. "If this were to occur, an attacker
who had access to an unencrypted SOAP message could potentially take actions as
another valid user and perform invalid EC2 actions," according to the
advisory.
Amazon
said it would be generally difficult for attackers to obtain a presigned SOAP
request or a signed certificate, but admitted it is possible if the customer is
sending SOAP requests over plain HTTP connections instead of the more secure
HTTPS protocol. The researchers also disclosed cross-site scripting flaws that
would have allowed attackers to obtain the certificate, according to Amazon.
This
is not the first time the CBC mode in encryption protocols was targeted. Two
researchers last year developed a "padding oracle attack" to decrypt encrypted
cookies for Websites and hijack users' secure sessions. The technique affected
the security of Microsoft's ASP.NET framework and forced an emergency patch
from Microsoft to close the hole.